全文获取类型
收费全文 | 94525篇 |
免费 | 1928篇 |
国内免费 | 1509篇 |
专业分类
测绘学 | 2465篇 |
大气科学 | 7443篇 |
地球物理 | 19548篇 |
地质学 | 32444篇 |
海洋学 | 8268篇 |
天文学 | 20810篇 |
综合类 | 370篇 |
自然地理 | 6614篇 |
出版年
2021年 | 784篇 |
2020年 | 874篇 |
2019年 | 940篇 |
2018年 | 1708篇 |
2017年 | 1670篇 |
2016年 | 2210篇 |
2015年 | 1571篇 |
2014年 | 2244篇 |
2013年 | 4724篇 |
2012年 | 2602篇 |
2011年 | 3702篇 |
2010年 | 3209篇 |
2009年 | 4449篇 |
2008年 | 3942篇 |
2007年 | 3741篇 |
2006年 | 3756篇 |
2005年 | 3014篇 |
2004年 | 3062篇 |
2003年 | 2812篇 |
2002年 | 2667篇 |
2001年 | 2372篇 |
2000年 | 2304篇 |
1999年 | 1971篇 |
1998年 | 1996篇 |
1997年 | 1921篇 |
1996年 | 1650篇 |
1995年 | 1600篇 |
1994年 | 1424篇 |
1993年 | 1305篇 |
1992年 | 1252篇 |
1991年 | 1093篇 |
1990年 | 1328篇 |
1989年 | 1135篇 |
1988年 | 1017篇 |
1987年 | 1251篇 |
1986年 | 1074篇 |
1985年 | 1320篇 |
1984年 | 1569篇 |
1983年 | 1456篇 |
1982年 | 1349篇 |
1981年 | 1282篇 |
1980年 | 1146篇 |
1979年 | 1092篇 |
1978年 | 1117篇 |
1977年 | 1049篇 |
1976年 | 977篇 |
1975年 | 927篇 |
1974年 | 948篇 |
1973年 | 959篇 |
1972年 | 576篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
761.
762.
The spatial and seasonal distribution of the viable heterotrophic population and three heterotrophic physiological groups, i.e. ammonifyers, nitrate reducers and sulphate reducers, were studied in two reservoirs in the province of Vizcaya (Spain). Using indicative parameters, both reservoirs were classified as warm monomictics, one hypereutrophic and the other mesotrophic. A direct relation between the size of the bacterial populations and the trophic levels of the reservoirs was observed. Likewise, the distributions of the bacterial populations in the water column are basically different. In the hypereutrophic lake there is a direct relationship between the bacterial distribution and the physical-chemical environment of the reservoir that cannot be observed in the mesotrophic lake. 相似文献
763.
M. Bouchon S. Gaffet C. Cornou M. Dietrich J.P. Glot F. Courboulex A. Caserta G. Cultrera F. Marra R. Guiguet 《Journal of Seismology》2000,4(4):517-523
We found extensive evidence that the vertical ground accelerations produced during the largest shock (M = 6.0) of the 1997 Umbria-Marche earthquake sequence exceeded 1g in two areas close to the heavily-damaged villages of Annifo and Colle Croce. This evidence comes from the striking observation of thousands of freshly fractured and broken rocks and stones in these areas. Some of the broken stones lie isolated on soft detritic soil while others had been previously piled up, probably a long time agoto clear the fields for farming. The freshness of the cuts and fractures and the consistency of the observations for thousands of rocks and stones in these areas indicate that these rocks were thrown upwards during the earthquake, with breakage occurring at the time of impact. Ground motion calculations consistent with the static deformation inferred from GPS and interferometry data, show that the broken stones and rocks are found in the zone where the strongest shaking took place during the earthquake and that most of the shaking there was vertical. 相似文献
764.
J. W. Meek 《地震工程与结构动力学》1978,6(5):437-454
When subjected to major earthquakes, core-stiffened buildings may begin to tip. That is, the overturning moment on the core's footing becomes so large that the footing breaks contact with the ground and begins to rock. A method is described for including the effects of tipping in the analysis of multistorey core-braced structures. Curves are presented which summarize the maximum response to both pulse and earthquake excitations; these data are elucidated via a typical design example. By comparison to fixed-base behaviour, tipping greatly reduces the base shear and moment. This makes possible a more economical design. However, attention must be devoted to avoiding potential soil-mechanics problems associated with the wobbling behaviour of the tipping core. 相似文献
765.
J. D. LASKI 《Geophysical Prospecting》1978,26(1):48-81
A method to compute the parameters of a multilayered medium with dipping interfaces is described. The input data are time-distance curves of diving waves. The output data are the thickness of the layers, dips, and parameters characterizing the increase of velocity with depth. Three examples are presented. 相似文献
766.
The transient phenomena associated with induced polarization are very complex and it is necessary to have both a proper understanding of the physical processes and a good supply of mathematical aids in order to make any advance in the quantitative interpretation of I.P. surveys. The analogy between the classical electrical sounding method and the induced polarization sounding method has led a number of workers to interpret the results of the latter like the results of the former. To clarify this idea, it seems very interesting to calculate a mathematical model in order to know the form of the anomaly that can be probably expected. The case of a thin horizontal polarizable layer, with the same uniform and isotropic conductibility as the homogeneous background, has been studied in this paper. The result of the calculation for the case of a single current electrode and a single potential electrode is given. From these data, the theoretical curve of IP sounding using the Wenner electrode array is computed. The master curve is compared with experimental curves obtained on scale models. A good correlation between these curves is noticed. In addition, it can be concluded that the mathematical model is proving the validity of the experiments on interpretation scale models. 相似文献
767.
768.
It is shown that the so-called Kirchhoff-summation operator is of a very wide-band nature and even contains an evanescent part. As a consequence, discretization may cause serious aliasing errors, particularly for small extrapolation steps. It is proposed to use in all practical cases band-limited versions of the summation operator, the spatial cut-off frequency being determined by the spatial Fourier spectrum of the coherent noise. 相似文献
769.
D. J. Stom S. S. Timofeeva N. F. Kashina L. J. Bielykh S. N. Souslov V. V. Boutorov M. S. Apartzin 《洁净——土壤、空气、水》1980,8(3):203-211
By the example of some phenols and qninones, in aqueous solutions possibilities for the quantitative determination are tested separately and in the mixture. Methods for thin-layer chromatography and spectrophotometry were developed. There can be chromatographically deteeted between 5 and 80 μg of substance. Phenol and quinone concentrations of 1.5 · 10?3 … 0.5 · 10?4 M were spectrophotometrically investigated. 相似文献
770.
G. Bergametti A.L. Dutot J.P. Quisefit R. Vie Le Sage 《Journal of Volcanology and Geothermal Research》1983,15(4):355-367
A statistical analysis of volcanic aerosol based on 1,083 variables including concentration, grain-size and site variabilities establishes correlations between sites and among chemical elements.Variation curves of element concentration show parallel behaviour. High coefficients of correlation between anions and cations confirm associations between elements. 相似文献